The instant invention relates to novel drilling fluid compositions. More particularly, the instant invention relates to a novel water base drilling composition formulated with a unique additive system which results in a drilling mud having superior stability characteristics and improved noncorroding characteristics.
The use of a drilling fluid, commonly called a drilling mud, in drilling operations using the rotary drilling method is well known. The drilling fluid functions to lubricate and cool the drill bit so as to increase its useful life and also serves as a carrier for the cuttings made by the bit in drilling the borehole. The drilling fluid also supplies a hydrostatic head on the formation to counterbalance any pressures of liquids or gases which are encountered in the various strata through which the drill bit passes. Finally, the drilling fluid should function to create a plastering effect on the walls of the borehole to limit the flow of fluid from the borehole into the formation and thus leave the formation in as near to a natural state as is possible.
The drilling muds which are employed in a majority of drilling operations are of the water base type. The popularity of these muds results from the universal availability of water and their relatively lower cost when compared with oil base muds or invert emulsion type muds.
Conventionally, the water base muds comprise fresh or salt water, clay, weighting agents, and miscellaneous additives such as thinning agents, fluid loss agents, and the like. One of the more popular classes of thinning agents and additives for water base muds includes the metal lignosulfonates. These lignosulfonate compounds are derived from the spent sulfite liquor obtained in the sulfite process of wood pulping. Although these compounds demonstrate satisfactory thinning properties when used in the mud, they present serious problems because of their instability in the presence of elevated temperatures and pressures which are increasingly encountered in the deeper wells being currently drilled. At high temperatures, the lignosulfonate compounds tend to break down and lose their thinning ability. However, a side effect of the lignosulfonate breakdown appears to be the liberation of hydrogen sulfide or possibly other acidic and corrosive compounds which present problems of corrosion of the tools being used in the borehole and furthermore appear to cause hydrogen embrittlement of the steel tools and drill pipe employed in the borehole.
The use of the lignosulfonates has consequently been hampered not only by a deterioration in the drilling mud properties requiring constant retreatment of mud subjected to elevated temperatures, but furthermore has presented an appreciable problem with respect to corrosion and hydrogen embrittlement of the drilling tools employed with the mud.
In view of this history of experience with the lignosulfonates, it is apparent that there is a need for an additive system for a water base mud which accomplishes the thinning function while not resulting in the formation of by-products which have corrosive or other deleterious effects on the well tools used in the mud environment.